Research for spatial diversity using a multiple input multiple output (MIMO) system has been actively ongoing in order to overcome performance degradation due to channel fading of radio communication.
The MIMO system, including a transmitter and a receiver each having two or more antennas, provides advantages such as a high data transfer rate, a low error rate, and an increased channel capacity, and the like.
However, in spite of the advantages of the MIMO system, in general, it is not possible to implement the MIMO system in uplink due to restrictions such as the size, weight, hardware complexity, and the like.
As an alternative, a cooperative diversity scheme has been proposed. The purpose of the cooperative diversity scheme is to obtain the advantages of a space diversity gain, a reduction in an error rate, an increase in a channel capacity, and the like, of the space diversity of the MIMO system. To this end, in the cooperative diversity scheme, a transmitter and adjacent relay terminals share resources such as frequency band and the like to form a virtual MIMO system to allow terminals having a single antenna to obtain the advantages of the MIMO system.
In the cooperative diversity scheme, orthogonal channels or orthogonal codes are used between relay terminals in order to eliminate interference between the relay terminals.
First, in the scheme using an orthogonal channel, the entire given channels are divided by time and frequency axes, and the respective relay terminals are allocated to the respective channels. This scheme is advantageous in that reception signals of the respective channel can be decoded by directly combining them but has shortcomings in that the increase in the use of channels by the number of relay terminals leads to a reduction in spectral efficiency. In addition, when the configuration of participating relay terminals, such as the number of relay terminals, changes due to a movement, the channels used by the relay terminals are also inevitably changed, so the relay terminals must be ordered (or aligned) at every transmission, and the like, increasing overhead of signal transmissions.
Next, in the scheme using an orthogonal code, although the relay terminals simultaneously transmit signals, their orthogonality is guaranteed, so there is no unnecessary reduction in the spectral efficiency. However, in order to use determined channel codes, relay terminals must need to accurately restore signals and re-encode them, which results in an increase in complexity of the relay terminals. In addition, when a channel between a transmitter and a relay terminal (a source-relay channel) is unstable, the scheme using an orthogonal code cannot be applicable. In addition, in order to use determined channel codes, a determined number of relay terminals must participate, which can be hardly achieved in actuality due to a frequency movement of the terminals. In addition, when there is a change in the configuration of the relay terminals due to a movement of the terminals, overhead of signal transmissions such as a transmission of information regarding ordering of the relay terminals and information regarding a code design, and the like, increases. Also, when the number of relay terminals that can participate is less than the number of relay terminals required for the determined codes, the scheme using an orthogonal code cannot be applicable. Thus, a network entry/exit of the relay terminals is restricted.
Meanwhile, the cooperative diversity scheme includes a single relay terminal-based cooperative diversity scheme using a single relay terminal and a multi-relay terminal-based cooperative diversity scheme using a plurality of relay terminals.
First, in the single relay terminal-based cooperative diversity scheme, the entire available time slots are divided into two orthogonal time slots for transmission of the transmitter and the relay terminal. In this case, on the first time slot, the transmitter broadcasts a message signal to other terminals serving as relays and a receiver 300. On the second time slot, the relay terminal retransmits the message signal, which has been received from the transmitter, to the receiver 300 according to a relay scheme. In this case, as the relay scheme, amplify and forward (AF) and decode and forward (DF) schemes are largely used. In case of the AF scheme, the relay terminal amplifies the message signal, which has been received from the transmitter, to have a certain size, and retransmits the amplified message signal. The AF scheme is advantageous in that it can implement simpler hardware because there is no signal processing procedure resulting from a decoding operation, but has drawbacks that because noise added in the relay terminal is delivered to the receiver 300, performance is degraded in a low signal-to-noise ration (SNR) environment. Meanwhile, in case of the DF scheme, the relay terminal decodes a signal received from the transmitter, re-encodes it to generate a message signal, and transmits the generated message signal to the receiver 300. However, in the DF scheme, the relay terminal decodes the message signal from the transmitter, re-encodes it, and then transmits the same, which is, thus, less affected by noise, but hardware complexity increases due to the decoding of the message signal of the transmitter.
Meanwhile, the multi-relay terminal-based cooperative diversity scheme has a time slot structure in the form of a repetition code. The entire available time slots are divided by the number of transmitters and relay terminals. On the first time slot, the transmitter broadcasts a message signal to other terminals serving as relays and the receiver 300. Thereafter, each relay terminal sequentially retransmits the message signal from the transmitter according to a determined relay scheme on each allocated time slot. Research reveals that the multi-relay terminal-based cooperative diversity scheme achieves good performance as the number of relay terminals increases, but the length of a time slot allocated for the message signal is reduced to cause a problem in that a bandwidth efficiency is reduced.
A space-time coding relay scheme is another type of the multi-relay terminal-based cooperative diversity scheme. In the space-time coding relay scheme, the entire available time slots are divided into two orthogonal time slots for transmissions of the transmitter and the relay terminal. On a first time slot, the transmitter broadcasts a message signal to other terminals serving as relays and the receiver 300. On a second time slot, relay terminals restore the signal which has been received from the transmitter, space-time-codes the restored signals to have orthogonality between the respective relays, and re-transmits the coded signals to the receiver 300. The space-time coding relay scheme is effective for a bandwidth utilization and disadvantageous in that hardware complexity increases according to the space-time coding to obtain orthogonality and the space-time coding relay scheme can be applicable only to a particular number of relays.
Research regarding the existing cooperative diversity scheme has been largely conducted on protocols, coding schemes, relay terminal selection schemes, and the like, in case of a single relay terminal, or when the number of relay terminals is uniform or complexity is high and a frequency efficiency is not good in case of multiple relay terminals.
However, in an actual wireless communication network environment, terminals tend to frequently move, and because relay terminals can freely participate in a network or exit from the network, the mentioned related art cooperative diversity scheme can be hardly applicable.